The expression wellstring is used to refer to any string or well tubular used for operations in a borehole, such as drilling, logging, fluid transportation. The well string does not necessarily need to be tubular over its entire length. The well string can in particular be a drill string, and can include a well-drilling bit.
International patent application with publication No. WO 00/17488 discloses a system for drilling and logging of a wellbore formed in an earth formation, wherein a logging tool can be lowered in the wellbore from inside a drill string through a well drilling bit at the lower end of the drill string.
The known system comprises a well-drilling bit including a bit body provided with a passageway for the logging tool, and a closure element for the passageway in the form of an insert section at the bit face. The bit body is attachable to a tubular drill string at a drill-string side of the bit body, and the passageway extends from an opening at the drill-string side to the well exterior of the bit body. The closure element comprises a bit-connecting means in the form of a primary latching device for selectively connecting the closure element to the bit body, so as to selectively close the passageway.
The known system further comprises an auxiliary tool for manipulating the closure element, which auxiliary tool forms the downstream part of a logging tool string.
In the specification and in the claims the terms upstream, upper and downstream, lower are used in relation to the lowering of a tool into a borehole, so that upstream, upper is closer to the surface than downstream, lower.
The logging tool string of the known system is arranged so that it can pass from the attached drill string through the opening of the bit body at the drill-string side, along the passageway so that it can reach the closure element, when the closure element is connected to the bit body. The auxiliary tool comprises a tool-connecting means in the form of a secondary latching device for selectively connecting the auxiliary tool to the closure element. The secondary latching device is further so arranged that simultaneously with the latching of the auxiliary tool to the closure element, the primary latching mechanism is operated so that the closure element is unlatched from the bit body while remaining attached to the auxiliary tool.
The well drilling bit of the known system can be used for drilling operation, when the closure element is connected to the bit body. When it is desired to log the formation, drilling operation is stopped, and the logging tool string with the auxiliary tool at its lower end is lowered through the drill string into the passageway. The tool-connecting means (secondary latching mechanism) is connected to the closure element, and, simultaneously, the bit-connecting means (primary latching mechanism) is operated so as to release the closure element from the bit body. Then, the logging tool can be lowered into the wellbore ahead of the well drilling bit from where logging can be performed. After logging has been completed, the logging tool string can be pulled back into the drill string, so that the closure element is re-connected to the bit body and the auxiliary tool is simultaneously disconnected from the closure element.
The known system has the disadvantage of limited robustness. Fail-safe operation of downhole equipment is generally very important in well drilling operations. With the known system there is for example a risk that the closure element is not sufficiently connected to the bit body in a situation wherein the auxiliary tool is not fully connected to the closure element. This can happen during removal of the closure element, and also when it is attempted to re-insert the closure element after logging was completed. Consequently, the closure element could be lost in the wellbore. The latching mechanism disclosed in International Patent Application publication No. WO 03/004825, which was unpublished at the priority date of the present application, has the same disadvantage. U.S. Pat. No. 3,554,304 discloses a well-drilling bit with cutter elements that are lowered on a conventional wireline with overshot through the drill pipe and locked to the lower end of the drill pipe by a latching mechanism. The wireline can be pulled back to surface and lowered again to reconnect to the cutter element. For removing the cutter elements to surface, the latching mechanism is released by simply pulling the wireline.